A recent increase in the amount of communication data in radio communications has grown demand for, for example, base stations with greater transmission power. As a wide-band and high-efficiency microwave power amplifier, a Doherty-type power amplifier has received attention. The Doherty-type power amplifier includes a main amplifier (carrier amplifier) that amplifies an input signal and an auxiliary amplifier (peak amplifier) that amplifies an input signal when the input signal exceeds a certain level.
For example, for high-frequency, high-power amplifiers in base stations, Doherty-type power amplifiers (Doherty amplifiers) have been frequently used as final-stage power amplifiers. In other words, a Doherty amplifier can obtain high efficiency with a mobile communication system signal having a large difference between average power and peak power, and thus such a Doherty amplifier has been frequently used as, for example, a final-stage power amplifier in a base station.
The main amplifier operates as a class A or AB amplifier, whereas the auxiliary amplifier operates as a class B or C amplifier. For example, the main amplifier operates alone at low output power, whereas the auxiliary amplifier operates with the main amplifier at high output power.
However, when the auxiliary amplifier is turned on to start amplification at high power, the output phase of the auxiliary amplifier is considerably different from that of the main amplifier, leading to a large loss of power combination of the output power of the main amplifier and the output power of the auxiliary amplifier.
As described above, for example, as a high-efficiency high-power amplifier in a base station, a Doherty-type power amplifier has been used which includes a main amplifier that amplifies an input signal and an auxiliary amplifier that amplifies an input signal when the input signal exceeds a certain level.
However, when the auxiliary amplifier is turned on to start amplification at high output power, the output phases of the auxiliary amplifier and the main amplifier are considerably different from each other, causing a large loss of power combination of the output power of the main amplifier and the output power of the auxiliary amplifier.
To solve the problem, a technique is proposed to control the gate biases of a main amplifier and an auxiliary amplifier according to input power and reduce a loss of power combination of the output power of the main amplifier and the output power of the auxiliary amplifier. However, the control of the gate biases of the main amplifier and the auxiliary amplifier needs the provision of, for example, an orthogonal splitter, a detector, and a bias control circuit, resulting in a complicated and expensive circuit.
In the related art, various Doherty-type power amplifiers are proposed. For example, the gate biases of a main amplifier and an auxiliary amplifier are controlled according to input power in order to reduce a loss of power combination of the output power of the main amplifier and the output power of the auxiliary amplifier.    Patent Document 1: International Publication Pamphlet No. WO2008/012883    Patent Document 2: Japanese Patent No. 4210332